Future energy and capacity shortages will likely lead to reductions in energy consumption and increased cost. This, in turn, will demand a more efficient electric power distribution system than presently exists. In order to maximize efficiency, voltage and current sensors are necessary to determine the energy flow. For this application fiber optic sensors could be ideal because of their dielectric properties.
The electric power system can be divided into three subsystems:
1. The generation system, made up of plants where electricity is generated. PA1 2. The transmission system, consisting of the power lines, transmitting power from the generating plants, often over long distances, to the areas where it is used. PA1 3. The distribution system, carrying power from the transmission system to the individual consumers.
Within the distribution system, several voltages often exist. System voltages differ from country to country. Line voltage is defined as the voltage between the phase conductor and ground. The voltage reaching the ordinary consumer is relatively low (110 V in the U.S.A. and Canada, 220 V in much of Europe). However, these voltages are too low for distribution over more than a very short distance, given expected losses in that system. As a result, one or more intermediate voltage levels exist in the distribution system between transmission voltage and the low voltage which reaches consumers. These intermediate voltage levels are typically in the range of 10-20 kV, but values above and below this are also common.
Measured in terms of miles or kilometers of electrical wire, the distribution system is the largest part of the electric power system. It is therefore also the part where the greatest loss exists and where the largest amount of money can be saved if the operation of the system is optimized.
The distribution system is very branched. Often the direction of power transmission can be changed by altering the way the branching is done. Load distribution determines the optimal way of operating the system. If the distribution of loads was known all the time, the system could be operated more economically.
Unfortunately, the distribution system is today the part of the electric power system from which the least data is being recorded. Current and voltage are only monitored at a few locations in the system. If currents and voltages were monitored more widely, breakers could be installed which, under remote control, could change the way the system was branched. Voltages could be remotely controlled by regulating the transformer ratios. The whole system could be operated in a much more cost efficient way. Also, if a fault occurred, it could easily be located and the faulty part isolated, so that the rest of the system could maintain normal operation. This concept is called distribution automation.
A distribution automation system depends on a large number of sensors to measure voltage, current or other information at individual nodes in the distribution power grid. A communication network is also necessary to collect data from the sensors and transmit data to the actuators. The network could be operated from a central computer, or the intelligence could be partially distributed, with most of the control done locally and only the data representing the status of the system sent to a central place for monitoring.
Fiber optic sensors have a number of inherent advantages in high voltage systems. It is more correct to talk about a fiber optic sensing system than a fiber optic sensor. Fiber optic measurement systems can be made in several different ways, but generally can be described as follows: The measuring system consists of a light source, which launches optical power into the transmitter fiber. The transmitter fiber transmits the optical power to the sensor head where interaction between the light and the measured quantity takes place. From the sensor head, the light is launched into the receiver fiber, which returns the signal to the optical detector.
Electrooptic polymer, used as the interaction material in such sensors, can be produced in bulk form and in large quantities at low cost. However, natural electrooptic crystals will remain costly as crystal growth, location of the crystal axes and polishing are all very complicated tasks. The cost of optical fibers has become quite low, as have costs for LEDs and PIN photo diodes for fiber optic use. Driver electronics, which are mostly operational amplifiers and standard electronic components are typically low cost products. Traditional optical polarization components are expensive. However, for wavelengths in the visible range, it is possible to buy extremely low cost polarizers and 1/4 wave plates made of polymer. Many different types and qualities of polarizers and wave plates exist, over a wide price range. If electrooptic polymer could be made transparent in the visible wavelength range, the cost of optical polarization components could come down significantly. In recent years, new polarizers and wave plates have come on the market for the wavelength 820 nm. These new products seem to be of good quality, yet are reasonably priced. As demand for these components increases, prices should drop even further. Sensor accuracy depends very much on the optical polarization components, and stringent accuracy requirements may keep the prices of these components high.
Traditional voltage transformers and voltage dividers are very expensive when designed for high voltage levels. The high price is mostly due to high demands on the electrical insulation system at higher voltage levels. In the case of the fiber optic voltage sensor based on an electrooptic polymer, the electrical insulation between the high voltage and the electronics which monitors the signal, is less of a problem because optical fiber is a natural insulator.
A sensor designed to be installed with one electrode attached to ground and the other to high voltage potential would, of course, have to be designed to withstand a voltage higher than the peak phase voltage. However, as the sensor is sensitive to electric fields, it might be possible to operate it without direct electrical contact to ground. If the sensor is mounted between the phase conductor and a surrounding cylinder at a floating potential, the measurement would become sensitive to changes in ambient electric fields. This will probably not be a problem if high accuracy is not a demand. In addition, the demand on the insulation properties of the sensor would be much lower and the consequences of a sensor breaking down would be much less severe. Clearly, fiber optic voltage sensors of this kind, based on an electrooptic polymer, can be produced at low enough cost to be feasible for distribution automation.
The most relevant prior art known to the applicants is the disclosure in published PCT Application WO88/02131 to P. S. Ramanujam published 24 Mar. 1988 and entitled "A PROCESS FOR PRODUCING AN ELECTROOPTICAL MATERIAL HAVING CONTROLLABLE PROPERTIES, AND USE OF THE MATERIAL IN ELECTROOPTICAL COMPONENTS". The principal distinctions between the present invention and the Ramanujam disclosure are the application of an electric field in direct contact with the epoxy resin during the poling step herein and the use in the invention of electric current as a process control parameter to obtain the maximum electric poling field short of dielectric breakdown in the cured epoxy. Further Ramanujam does not disclose a voltage sensor of the precise configuration described herein. These and other distinctions make the invention disclosed and claimed herein patentably unique over Ramanujam. Other relevant prior art includes an article by M. A. Hubbard et al, appearing at pages 136-143, SPIE Vol. 971 (1988) and an article by Manfred Eich et al, appearing at pages 128-135 of the same publication. Both of these articles discuss the nonlinear optical properties of dye/crosslinked polymer systems. Other articles of relevance include:
1. K. Bohnert et al, pages 290-292, Optics Letters, Vol. 14, No. 5 (Mar. 1, 1989); PA0 2. K. D. Singer et al, pages 1800-1802, Appl. Phys. Lett. 53 (19), (Nov. 7, 1988); PA0 3. K. D. Singer et al, pages 248-250, Appl. Phys. Lett. 49 (5), (Aug. 4, 1986); PA0 4. Manfred Eich et al, pages 3241-3247, J. Appl. Phys. 66 (7), (Oct. 1, 1989).